Rsync is a synchronization tool for backup and more.
Lifehacker has a nice piece about rsync, one of my favorite backup utilities <http://lifehacker.com/software/rsync/geek-to-live-mirror-files-across-systems-with-rsync-196122.php>:
“You use more computers and operating systems to get your work done today than ever before. But how do you keep your files synchronized between them? Plenty of OS-specific tools can mirror folders, but you need something cross-platform and highly customizable. You need the 10-year-old command line file mirroring utility called rsync.”
For those who may not want to mess with the command line, DeltaCopy <http://www.aboutmyip.com/AboutMyXApp/DeltaCopy.jsp> is an open source rsync client and server for Windows with a graphics user interface (GUI).